One purpose of electrochemically etching is to increase the surface area of a metal foil. Since the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, which are often aluminum foils, increasing the surface area of an aluminum foil is useful to increase the capacitance of an electrolytic capacitor. One type of electrochemical etching process increases surface area by removing portions of the aluminum foil to create etch tunnels. Typically, etch tunnels are created by first making the aluminum foil anodic in an electrolyte, and then passing an electric current between the anode and cathode.
Metal foil is commonly pretreated (treated prior to etching) in order to maximize the increase in surface area and improve the distribution of etch tunnels during the subsequent etching steps. A pretreatment can be one of three types: mechanical, chemical, or electrochemical. A mechanical pretreatment strokes the surface of the metal foil with a high speed rotating metal brush to remove a surface layer and uniformly texture the surface of the foil. This type of mechanical pretreatment is an old practice in the art.
A chemical pretreatment removes residual processing oils from the surface of the metal foil and dissolves surface oxides, or replaces the surface oxides with a new surface film. Commercial cleansing agents, acid solutions, or alkaline solutions are commonly used to remove surface oils and/or dissolve surface oxides. An example of a method to replace the surface oxide with a new surface film is disclosed in Japanese Patent No. 60,163,426 [85,163,426] (CA 103:204566u), which teaches the use of a pretreatment of chromic acid prior to electrochemically etching aluminum foil. This chemical pretreatment changes a film on the surface of the foil from aluminum oxide to a mixture of aluminum oxide and chromic oxide.
An electrochemical pretreatment removes a relatively small amount of the surface metal during an initial electrochemical etch step, when compared to the amount of surface metal removed during the subsequent primary electrochemical etch step. U.S. Pat. Nos. 4,437,955 and 4,676,879 show examples of electrochemical methods of pretreatment.
Several issued patents disclose methods of physically depositing metal onto metal foils in order to enhance the resulting capacitance of the foil. Japanese Patent No. 63,100,711 [88,100,711] (CA 109:84667c) discloses the chemical vapor deposition of titanium onto a previously electrochemically etched aluminum foil. Japanese Patent No. 63,255,910 [88,255,910] (CA 110:106582w) discloses multiple layers of titanium deposited by solvent evaporation onto a previously etched aluminum foil. Japanese Patent Nos. 03 06,010 [91 06,010]; 03 32,012 [91 32,012]; and 03 30,410 [91 30,410] (CA 114:198072p; CA 115(2)20506r; and CA 115 (2)2025q, respectively) disclose methods of depositing titanium, gold, and platinum onto aluminum foil by cathode arc evaporation. German Patent No. 27 58 155 teaches a method of preparing a corrosion-resistant electrolytic capacitor anode by using evaporation or sputtering methods to deposit a tantalum film onto aluminum foil. The deposited film must be continuous over the surface of the foil, i.e., greater than a monolayer in thickness, in order to provide corrosion-resistance for the underlying foil.
Two patents show pretreatment methods of depositing metal onto a surface of an aluminum foil prior to etching the foil. Japanese Patent No. 01,283,812 [89,283,812] (CA 112:228258g) teaches a method of preparing aluminum foil for cathode use in a capacitor. The foil is pretreated by surface deposition of a metal alloy film containing low corrosion-resistant and high corrosion-resistant metals, with examples of the high corrosion-resistant metal being titanium or chromium. The foil then is chemically or electrochemically etched to remove the low corrosion-resistant metal, thus increasing the surface area of the foil while leaving the high corrosion-resistant metal on the foil surface. Japanese Patent No. 02 61,039 [90 61,039] (CA 114:73672c) also teaches a method to prepare aluminum foil for use in an electrolytic capacitor. The foil is pretreated by surface deposition of a valve metal, followed by ion etching to increase the surface area of the foil. This method is limited to using a valve metal for the pretreatment deposition, and the deposited layer must be thicker than one monolayer in order to subsequently ion etch the valve-metal-coated surface.